Protoss
The lowest-cost Protoss unit is 100 minerals. That's twice the cost of a Marine, and four times the cost of a Zergling, though Zerglings also cost Larvae, a resource other races don't use.

This is a fundamental flaw, which affects Protoss's entire game. It's part of the reason the Mothership Core is necessary. The Mothership Core is an unfortunate byproduct of a bad design.

The base Protoss Unit should cost no more than 75 minerals. And I think it should be a flying saucer! It would get crushed by Marines and be positionally useful in warding off Zerglings. Also it could scout.

I think the Zealot should be a late-game unit. I think they should come with Charge. I think they should be enabled by the Twilight Council or enabled by researching Charge. They should be more expensive, faster, and more damaging.

I think all Protoss buildings should be balls of energy, and thus have all of their hit points in shields. I think Photon Overcharge should be a feature of all Nexus, should cost minerals as well as energy, and should increase shield recharge rate. I think Pylons should heal (friendly) Protoss units, and I don't think Pylons should allow unit warp-ins.

SHOULDS NEVER COULD.

I think Time Warp should be a power of High Templars, or maybe Tempests. I think recall should be a power of Carriers, and there should be no Mothership. A Protoss Mothership would cover the entire board! That's the only way Protoss makes sense.